Gas permeation tube and method for the filling thereof

ABSTRACT

A first permeation tube embodiment has a housing with exterior circular grooves at one end thereof. A gas permeable, Teflon membrane is fitted over the grooved end and an annular retaining ring is then press fitted over the grooved end to create a tortuous seal. An opposite end of the housing contains a needle valve inlet through which a precisely metered volume of gas is introduced by a gas transfer technique. A second permeation tube embodiment includes a container from which liquified gas flows to envelop the exterior surface of a tubular membrane. Air flows through the membrane to dilute gas permeating inwardly through the membrane.

. United States Patent 1 91 Budd et a1. Jan. 29, 1974 [54] GASPERMEATION TUBE AND METHOD 3,679,133 7/1972 Sekiguchi et a1. 239/34 FTHE FILLING THEREOF 3,111,091 11/1963 Hopkinson 239/34 X [75 1Inventors: fi iz ggg g gag h Primary Examiner-Lloyd L. King {:iathan S.Greenberg, Arlington, ABSTRACT [73] Assignee: Monitor Labs Inc., SanDiego, Calif. A first permeation tube embodiment has a housing withexterior circular grooves at one end thereof. A [22] Flled 1971 gaspermeable, Teflon membrane is fitted over the [21] Appl. No; 212,272grooved end and an annular retaining ring is then press fitted over thegrooved end to create a tortuous [52] U S Cl 239/34 239/56 seal. Anopposite end of the housing contains a needle [51] m0 9/04 valve inletthrough which a precisely metered volume [58] Fie'ld 239/34 56 of gas isintroduced by a gas transfer technique.

A second permeation tube embodiment includes a [56] References Cicontainer from which liquified gas flows to envelop UNITED STATESPATENTS the exterior surface of a tubular membrane. Air flows 3 412 93511/1968 OKeffe- 239/34 through the membrane to dilute gas permeating3:623:65) 11 1971 Maierwn ei all: 1:: 239/56 mwardly through themembrane 3,283,787 11/1966 Davis; 239/34 6 Claims, 8 Drawing Figures GASPERMEATION TUBE AND METHOD FOR THE FILLING THEREOF BACKGROUND OF THEINVENTION The present invention relates to a gas dispensing device, andmore particularly to a sealed vessel containing a gaseous substance inequilibrium with its liquid phase and having a permeation membranethrough which gas can pass and be diluted by another gas.

BRIEF DESCRIPTION OF THE PRIOR ART In the past, permeation tubes havebeen popularly used as standards for generating gas mixtures. Thesetubes are usually made by filling a vessel having at least a portionthereof made from FEP Teflon (Du Pont) tubing having known size andcharacteristics. Permeation of the gas through the Teflon generates asmall concentration of this gas into a diluent gas which is generallyair. In the area of pollution instrumentation, the permeation tube isemployed to generate a small quantity of precisely measured purepollutant gas where the pollutant gas is stored in the permeation tube.

Typical permeation tube designs are disclosedin U.S. Pat. No. 3,412,935to OKeeffe. Although'conventional tubes, such as disclosed in thispatent, work satisfactorily for certain applications, they suffer fromseveral disadvantages.

Initially, it must be pointed out that for use in pollutioninstrumentation, permeation tubes must be designed to produce a permeategas that when diluted with purified air (zero gas), an atmosphericpollutant is simulated. However, the prior art devices are constructedwith a relatively large permeating surface so that'a small concentrationvalue cannot be obtained easily. 1

Further, prior art permeation tubes have suffered from inadequate seals.As a result, these tubes not only permeate the gas contained therein,but they also leak.

As to the conventional method of filling permeation temperature to allowtransfer of liquid gas from a large storage cylinder to the permeationtube. However, it

has been found that very cold temperatures cause a Teflon membrane tocrack due to brittleness.

Because of these and other problems of the prior art permeation tubes,it is highly desirable to improve existing permeation tubes so thatsmaller concentrations of permeate gas can be obtained. Also, animproved filling method is desirable which would allow the transfer ofgas into a permeation tube kept at a temperature sufficiently high toprevent cracking of a Teflon membrane.

BRIEF DESCRIPTION OF THE INVENTION The present invention is a markedimprovement of presently existing permeation tubes. Further, there isdisclosed a novel method for filling a permeation tube such as the onedescribedherein.

The present permeation tube includes a permeating membrane that can bemade controllably small so that a pollutant gas of small concentrationis formed.

The invention includes a tortuous seal between the Teflon membrane and avessel containing the penneand gas. This seal eliminates leakage of thepermeand tubes, a permeation tube is usually maintained at a low whichwould add an error factor to the permeation rate.

By employing a needle valve within the structure of the permeation tube,the presently disclosed permeation tube can be reliably filled at a highenough temperature to prevent the cracking of the permeation Teflonmembrane.

The above-mentioned objects and advantages of the present invention willbe more clearly understood when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating the internal structure ofone embodiment of the permeation tube disclosed in the presentinvention.

FIG. 2 is a diagrammatic illustration of a method for filling thepermeation tube of FIG. 1.

FIG. 3a is a partial sectional view illustrating a modification of theend portion of the permeation tube shown in FIG. 1.

FIG. 3b is a partial sectional view illustrating a second modificationof the permeation tube structure shown in FIG. 1.

FIG. 4a is a sectional view illustrating a second embodiment of thepresent invention.

FIG. 4b shows an additional modification of the structure shown in FIG.4a.

FIGS. 4c and 4d show further modification of the permeation tubestructure shown in FIG. 40.

Referring to the figures and more particularly FIG. 1, reference numeral10 generally indicates a first permeation tube embodiment. Thepermeation tube includes a hollowed housing 12 having a centralcylindrical body terminating outwardly in an end portion 14 that hascircular (non-helical) grooves formed therein. Because of the corrosivenature of the substance contained in the permeation tube 10, it ispreferable that the material from which the tube is manufactured behighly corrosion resistant, for example stainless steel.

A membrane 16 is slipped over the grooved end 14 of the housing 12. Byway of example, the membrane is indicated as being cup-shaped. Themembrane must be permeable to the enclosed gaseous contents (permeand)of the tube. In a preferred embodiment of the invention, the membrane 16is made from FEP Teflon (Du Pont). The outward closed end 18 of themembrane 16 forms a membrane closure for the axially formed bore chamber22 that contains liquid gas 24.

When assembling the permeation tube, the membrane 16 is slipped over thegrooved end 14 and thereafter, a retainer ring 20 is force fitted overthe membrane. This causes the Teflon material to cold flow into thegrooves to effect a tortuous seal. This seal is far superior to thepermeation tube seals of the prior art.

Above the liquid gas column 24 is the gaseous phase 26. The liquid andgaseous phases are in equilibrium. Reference numeral 28 characterizesthe permeation path of gas molecules.

The permeation tube has an enlarged, generally cylindrical head 32containing a needle valve assembly.

An axial bore portion 34 is formed in axially spaced relation to theaforementioned bore chamber 22. Threads are formed along the interiorsurface of the bore 34 to accommodate a valve stem 44 therein.

Another bore 36 is formed in perpendicular, intersecting relation withthe aforementioned threaded bore 34. The bore 36 has an inlet port 38which engages a fill tube during filling of the permeation tube, asdiscussed hereinafter. The valve stem 44 allows filling of the tube whenthe stem is unscrewed, and after filling is completed, the valve stem istightened to shut off the needle valve in the permeation tube head 32.As will be observed, the inward end of the bore 36 terminates in atapered end 40 which receives the mating end of a fill tube (not shown).A restricted, narrow passage 42 connects confronting ends of the bores34 and 22. This passage is adapted to receive the inward tapered end 50of the valve stem 44. As will be observed, the tapered end 50 of thevalve stem is flat at 52, and a cylindrical intermediate portion 48connects the tapered end portion 50 and an enlarged cylindrical headsection 45 that terminates outwardly in a slot 46, adapted to receive ascrewdriver.

An O-ring 54 surrounds the inlet opening 38 in bore 36. This O-ringserves to seal the interface between the permeation tube and a separatefill tube during the filling of the permeation tube. An O-ring 56 isalso provided at the outward end of bore 34 to seal the interfacebetween the valve stem 44 and the bore 34.

Equivalent means for clamping the membrane 16 to housing 12 includes theformation of threaded grooves at the transverse outer end of the housinginstead of the grooves where illustrated. A retainer ring having matingthreads would then clamp the membrane against the outer end of thehousing.

FIG. 2 diagrammatically illustrates a method and apparatus for fillingthe permeation tube 10. The tube 68 are closed. When the permeation tubevalve 44 is opened, gas equilibrates in the system and most of theliquid contained in the chamber 76 is transferred to the permeation tube10. The transfer of liquid gas from chamber 76 to the permeation tubeincorporates a change to the gaseous phase along ,the path betweenchamber 76 and the permeation tube 10. However, due to the packing ofthe permeation tube 10 in the ice bath 60, the gaseous phase is cooledand again changes to the liquid phase once inside the permeation tube.After the gas has been transferred out of chamber 76, the valve 44 inthe permeation tube 10 is closed. The ice bath 60 is removed and thenthe permeation tube 10 is disconnected from the filler tube 62. Now, thepermeation tube 10 is readyfor use. When the filled permeation tube isremoved from the filler tube 62, the conduit path between valve 78 andthe outlet end of filler tube 62 are filled with air. However, theportion of the system from valve 78 back to valve 82 does not contain ispositioned in a temperature bath vessel 58 containing crushed ice. Themembrane 16 is thereby kept at freezing temperature which is notsufficiently low to cause cracking of the Teflon material. However, thefreezing temperature is cold enough to maintain gas, in the permeationtube, in the liquified phase. As will be observed from the figure, theenlarged valve portion of the permeation tube 10 rises above the icepack 60 and is exposed to allow connection of a filler tube 62 into theinlet port 38 of the permeation tube. The opposite end of the fillertube 62 is connected to a T-connector 64. A second leg of theT-connector is fastened to tubing 66 having a shutoff valve 68 along itslength. The outward end of the tubing 66 terminates in a vacuum pump 70.The third leg of the T-connector 64 is connected to a coupling 72 thatis in turn connected to a shutoff valve 74. The opposite end of thevalve 74 communicates with a chamber 76 that is bounded at its oppositeend by a shutoff valve 78. The volume of the chamber 76 is important.The volume is intentionally designed to be substantially equal to thevolume of the liquid column in bore 22 of the permeation tube. Thus,chamber 76 serves as a metering device. As will be explainedhereinafter, the metered volume of liquid gas filling chamber 76 istransferred to the permeation tube 10.

Tubing 80 is connected between the shutoff valve 78 and another shutoffvalve 82. An inverted fill tank 84 containing liquified gas thereindirectly communicates with the valve 82.

In operation of the system illustrated in FIG. 2, valve 82 is closed andvalves 78, 74, 68 and 44 are opened. Then, the vacuum pump 70 isoperated to evacuate the entire system, including the permeation tube,of air and water vapor. Next, valve 74 is closed and valve 82 is openedthereby resulting in the flow of liquid gas from tank 84 into chamber 76via tubing 80. Valves 78 and air. Accordingly, when the next permeationtube is positioned in place against the outlet end of filler tube 62,the permeation tube, and the conduit portion between valve 78 and theoutlet end of filler tube 62 must be evacuated by the vacuum pump 70.This requires that evacuation take place at the beginning of the nextfilling cycle. Thereafter, valves 68 and 74 are closed and valve 78 isopened and liquid gas flows to fill the chamber 76. From this point inthe procedure, the aforementioned filling steps are repeated.

As will be appreciated, the filling of the permeation tubes areconducted under relatively high pressure. Also, since the membrane 16 ismaintained at a high enough temperature to prevent cracking, a highdegree of quality control for the membrane can be realized.

Although the preceding discussion has been with respect to a cup shapedmembrane 16, this particular shape is noncritical and other designs arefully in keeping with the invention.

FIG. 3a illustrates the permeating end of a permeation tube similar tothe previously discussed permeation tube 10. However, the illustrationdepicts an elongated test tube shaped membrane that is sealed betweenthe retainer ring 86 and the grooved end 88.

FIG. 3b illustrates a further variation wherein a hollow cylindricalpermeating membrane 92 is fastened to the housing 94 of the permeationtube as in previous embodiments. However, a stainless steel or othernoncorrosive plug 96 is inserted in the outward end of the permeationmembrane 92.

An alternate embodiment of the present invention is shown in FIG. 4awherein a permeation tube assembly is generally denoted by 98. Theassembly includes an inverted container 100 having a fill plug 102therein. It should be understood that any suitable means for filling thecontainer 100 is permissible. Accordingly, the needle valve ofpermeation tube 10 can be used instead of the fill plug 102. Inside thecontainer 100 is a desired permeand in the gaseous phase 106 and theliquid phase 104, that are in equilibrium with one another. Thecontainer includes an elongated neck portion 108 that has a centrallyformed bore therein which communicates directly with the interior of themain container body. A leg 110 of a T-connector 112 has a bore 114formed therein for receiving the neck portion 108 of the container 100.A swagelock fitting 116 secures a seal between the container 100 and theleg 110 of the T-connector 112. This leg of the T-connector includes anaxially formed passageway 118 communicating with the container 100. Thepassageway 118 has a lower annular portion 120 that is positioned incentral coaxial relationship to the other legs of the T-connector 112. Acentral bore 122 is formed through these horizontally illustrated legs.A tubular permeation membrane 126 is positioned in the bore 122 andswagelock fittings 128 and 130 seal the exterior ports of the horizontalT- connector legs to the permeation membrane 126.

Thus, in operation of the device, when liquid permeand 104 flows throughthe container neck 108 passageway 118, annular passageway 124,-and bore122, the liquid gas envelops the membrane 126 and is contained betweenthe swagelock seals 128 and 130. Thereafter, the liquid permeandpermeates radially inwardly to the inside of the permeation membrane126. As zero gas flows axially through the permeation membrane 126, thezero gas mixes with the generated permeate to form a gas mixture thatexists from the outlet end of a permeation member 126.

FIG. 4b illustrates a further variation employing permeation of theliquid permeand in aradially inward direction. In this embodiment, acontainer 131 has a neck portion 132 that communicates with acylindrical housing 134 having axially projecting annular flanges 136and 138. A tubular permeation membrane 144 passes through the flanges136 and 138. O-ring seals 140 and 142 seal the confronting surfaces ofthe flanges 136, 138 and the permeation membrane 144. The gas flow is asindicated by arrows.

Still anotherv embodiment is illustrated in FIG. 40 wherein thecontainer 146 has its neck portion 148 extending into a cylindricalhousing 150 that has transverse flattened ends 152. Axial openings 154and 156 are formed in the transverse ends 152. O-rings 158 and 160 sealthe confronting surfaces between the flattened housing ends 152 and thecircular flanges 170 of conduit fittings 164 and 162. A short length oftubular permemation membrane 166 is fitted to nipples 168 that extendaxially inwardly and are an integral part of the fittings 162 and 164.

A still further variation of the radially inward permeating device isillustrated in FIG. 4d. In this embodiment, a container 172 includesgaseous and liquified gas in equilibrium as in-the precedingembodiments. A container neck 174 extends'into a cylindrical housing176. Gas tight bulkhead fittings 178 and 180 extend axially through thetransverse ends of the cylindrical housing 176. A tubular permeationmembrane 188 is axially positioned to pass through the bulkhead fittings178 and 180 as well as the cylindrical housing 176. Swagelock fittings186 prevent the escape of gas through the interface between thepermeation membrane 188 and the swagelock fittings 186. As in the 7 caseof the lastthree discussed embodiments, air flows into the permeationmembrane at the left end thereof and as it passes to the left outlet endof the membrane, the contained permeand permeates radially inwardly intothe membrane 188. After the gas enters the membrane, it is diluted bythe zero gas flowing through the permeation membrane 188 and isdelivered to the outlet end of the membrane.

With the exception of FIG. 3b, each of the illustrated and describedembodiments exclude conventional end plugs for the permeation membrane.Elimination of these plugs is advantageous for several reasons. First,excluding end plugs eliminates the hazard of plug ejection at hightemperature. Second, the seals of all embodiments of the presentinvention provide more positive gas sealing then end plugs.

In the instance of all embodiments presented herein, the permeationtubes eliminate short lifetime due to small fill volumes. The proposeddevices can be recharged without adjustment of permeation membranelength. Also, use of a stainless steel housing and container providesmore constant heat sinking for contained liquids than a permeation tubealone. As an extension of this advantage, the constructions hereinprovide rapid temperature change for better control due to the highthermoconductivity of the housing of permeation tube 10. In theembodiments of FIGS. 4a-4a', a gas containing container adds to the highthermoconductivity characteristic.

Accordingly, from the aforementioned specification and accompanyingclaims it can be appreciated that the present invention offers distinctadvantages over the prior art permeation tubes.

It should be understood that the invention is not limited to the exactdetails of construction shown and described herein for obviousmodifications will occur to persons skilled in the art.

Wherefore, the following is claimed:

1. A permeation device comprising:

a casing for storing liquid gas therein;

an outer end of the casing having grooves therein;

a permeation membrane positioned over the grooved end of the casing; and

fastener means clamping the membrane against the grooved end of thecasing for causing cold flow of membrane material in the grooves andcreating a tortuous seal between the membrane and the grooved end of thecasing.

2. The structure recited in claim 1 wherein the permeation membrane iscup-shaped and includes a cylindrical wall bounded on one transverse endby a circular integral closure.

3. The structure as set forth in claim 1 wherein the permeation membraneis characterized as test tube shaped with an open end and an oppositelydisposed oblong closed end.

4. The structure as defined in claim 1 wherein the permeation membranehas a hollow tubular shape having opened ends, and further wherein asealing plug is inserted in an outlet end of the membrane.

5. A permeation device comprising:

a container having liquified gas;

a permeation membrane positioned adjacent the container; and

conduit means having a diluent passing therethrough, the conduit meanscommunicating between the container and the permeation membrane tochannel flow of the liquified gas around the exterior of the membrane;whereby the gas permeates from the outside of the membrane to the insidewhere it is mixable with the diluent.

6. The structure of claim 1 wherein the membrane is composed of apolymeric plastic material.

1. A permeation device comprising: a casing for storing liquid gastherein; an outer end of the casing having grooves therein; a permeationmembrane positioned over the grooved end of the casing; and fastenermeans clamping the membrane against the grooved end of the casing forcausing cold flow of membrane material in the grooves and creating atortuous seal between the membrane and the grooved end of the casing. 2.The structure recited in claim 1 wherein the permeation membrane iscup-shaped and includes a cylindrical wall bounded on one transverse endby a circular integrAl closure.
 3. The structure as set forth in claim 1wherein the permeation membrane is characterized as test tube shapedwith an open end and an oppositely disposed oblong closed end.
 4. Thestructure as defined in claim 1 wherein the permeation membrane has ahollow tubular shape having opened ends, and further wherein a sealingplug is inserted in an outlet end of the membrane.
 5. A permeationdevice comprising: a container having liquified gas; a permeationmembrane positioned adjacent the container; and conduit means having adiluent passing therethrough, the conduit means communicating betweenthe container and the permeation membrane to channel flow of theliquified gas around the exterior of the membrane; whereby the gaspermeates from the outside of the membrane to the inside where it ismixable with the diluent.
 6. The structure of claim 1 wherein themembrane is composed of a polymeric plastic material.